The long term objective of this research is to understand how cells adapt to changes in oxygen concentration. This research is important because knowledge of how cells sense oxygen in their environment may provide insight into the mechanisms of disease states that are caused by oxidative stress and/or hypoxia. Experiments will be carried out on the FNR transcription factor, an oxygen sensing iron-sulfur (Fe-S) protein that is found in E. coli. The specific aim of these experiments is to understand how the [4Fe-4S] FNR protein and its aerobic degradation products, the [2Fe-2S] and apoprotein forms, act as an efficient oxygen sensing system. It is hypothesized that the presence of the [4Fe-4S] cluster alters the conformation and the dimerization properties of the protein, which leads to the formation of an active transcription factor. To determine if dimerization is the key to regulation of FNR, the dimerization constants of the different forms of FNR will be measured quantitatively by sedimentation equilibrium or elution gel chromatography. The different forms of the protein also will be evaluated by circular dichroism spectroscopy and protease sensitivity experiments to determine how the presence and type of Fe-S cluster alters the protein conformation. The communication between the dimerization domain and Fe-S cluster will be analyzed by site-directed mutagenesis and through the isolation of suppressor mutants. Through the combination of these experiments, the complete picture of the structure/function relationship in FNR will be obtained. The concepts ascertained from these experiments may be widely applicable to oxygen sensing in other cell types.